Empirical analytic method and theoretical analytic method are usually applied to calculate and predict water gushing yield in the tunnel engineering in the past but have much limitations and errors. Ample water source, hydrodynamic conditions and expedite channels are essential for water-bursting accidents in railroad tunnel engineering. From a lot of study of fractal geometry in rock mass, it is found that the multi-dimension value of fissure network is colligate reflection of the length, the direction, space and the number of fissures, so the value can be used to describe the complicated frameworks in rock mass. But it is a sound tackle to quantities the complicated frameworks. Numerical simulating method is convinced to be applied to calculate and predict water gushing yield of tunnel and underground engineering. It focuses on analyzing texture characteristics of water-containing medium, building hydro-geological conceptual model, and modeling long-term hydrodynamic field development. An experimental application in Paoziling tunnel under-crossing mountain reservoirs in Hunan province was presented in this article, and long-term hydrodynamic field development had been obtained under the condition of drainage from the tunnel, and the gush yield in the tunnel during the process of storing water in the reservoir was also predicted.